Quantifying the changes in the Earth's climate and determining if the changes result from natural variability or have an anthropogenic origin are fundamental to scientific and public policy. The climate change variables of interest are derivable from passive space-borne measurements in the wavelength range from the ultraviolet into the thermal infrared. These atmospheric variables include temperature profiles, water vapor, ozone, aerosols, precipitation, CO2 and other greenhouse gases; surface variables include vegetation, snow cover, sea ice, sea surface temperature, and ocean color. Solar spectral and total solar irradiance measurements are required to retrieve the needed climate parameters. These measurements contribute to the Earth radiation budget, the accounting of the incoming solar radiation, the re-radiated portion, and the trapped fraction responsible for climate change. Currently on-orbit instrumental uncertainties are comparable to or substantially greater than the desired climate change geophysical signals, and they occur on time scales that are at the operational lifetime of typical satellite systems that perform the essential measurements. Stable long term on-orbit calibration is needed for these measurements to separate physical process related signals from those that are instrumental in origin. Climatologists recognizing this have called for space based calibration traceable to the International System of Units (SI) with an accuracy that assures the fidelity of long term trending between generations of sensors.
Historically, reflective or transmissive solar diffusers have been used for top of the atmosphere calibration of atmospheric radiances. Solar radiation scatters from a diffuser into the entrance aperture of the remote sensing instrument to be calibrated. The diffuser transforms the solar irradiance into a top of the atmosphere radiance source that is used to calibrate Earth spectral radiance measurements. Use of a NIST standard of spectral irradiance combined with an accurate measurement of the diffuser bidirectional reflection distribution function (BRDF) calibration allows the accurate calibration of the sensor. This approach has long been the accepted method for calibrating atmospheric and surface radiance sensors. However, this methodology has been plagued by unaccountable changes in the diffuser properties. Therefore, by itself the diffuser technique does not provide the long term stability needed for climate trending on-orbit.
Several techniques have been proposed, explored, and implemented on-orbit to overcome the diffuser problem. For example, terrestrial radiance measurements from high spectral and high spatial resolution imaging instruments have been referenced to lunar irradiance measurements for determining the long term effects of on-orbit instrument degradation. Using lunar irradiance for on-orbit calibration has proven to be insufficient for climate change detection. In particular, the lunar spectral radiance is rapidly changing and non-uniform; however the lunar spectral irradiance is described by the Robotic Lunar Observatory (ROLO) model as a function of lunar phase angle that has a relative uncertainty of approximately 1.0%. The problem of variable radiance is compounded by detector spatial non-uniformity coupled with a wavelength-dependent responsivity; consequently, high spatial resolution instruments require an integration of the Lunar spectral radiance. The absolute albedo of the Moon is known with an uncertainty of about 5%. This limits its usefulness as an absolute spectral albedo standard in space; however, it is an excellent relative spectral albedo reference in space. Through modeling and measurement this uncertainty can be reduced and inter-calibration of satellite sensors will benefit. Vicarious calibration offers another approach to long-term calibration stability. Using the vicarious calibration approach, ground truth measurements of selected terrestrial areas exhibiting a high degree of spatial and spectral uniformity are periodically measured as spectral radiance targets for high spatial resolution instruments in space. The uncertainties of this technique limit its accuracy to roughly 3%. A third method, simultaneous nadir overpass (SNO) for inter-satellite calibration has been developed and applied to sensor calibration. A relative calibration accuracy of 2 to 3% has been achieved between instruments. There are plans to support other instruments with the SNO technique.
Signal attenuation methods have been used to observe and quantify the total solar irradiance. In signal attenuation approaches, calibrated apertures and beam attenuating filters are inserted into the optical path to attenuate the solar signal. In this spirit, a spectrometer using precision apertures, optical attenuators, and variable integration times to measure the solar irradiance while directly viewing the Sun is being developed. The apertures and attenuators from the beam path are removed and the integration time is changed to view the Earth. The solar and Earth views do not have a common instrumental optical path. When the solar and Earth measurements are divided, common path uncertainties propagate. This variable optical path method suffers from the unavoidable uncertainties associated with each signal attenuation step.
While various approaches have been proposed or deployed, stable on-orbit calibration that assures high confidence in detecting changes in climate related variables remains unrealized.